This invention relates generally to a method and apparatus for positioning objects for microscopic examination and, more particularly, to apparatus that attaches to a standard microscope to enable parcentric positioning of objects.
A standard microscope includes a light source, an objective lens and viewing arrangements (e.g. eyepiece or camera/CRT) for examination or manipulation of objects at high magnifications. Standard microscopes include commercially available petrographic, metallographic and semiconductor microscopes.
Parcentric positioning is the positioning of complementary points on two different objects at the focal point of the objective lens on a microscope. Complementary points refer to any pair of points on two objects that are related by a mirror plane. Such points would be in contact if the two objects were placed in a face-to-face orientation.
The need for parcentric positioning arises in several fields of scientific research, one of which is a scientific technique known as fission-track dating. This technique, which is used too determine the time of cooling of rocks in the earth's crust, involves the analysis of microscopic damage zones in certain uraniferous minerals that are produced by the spontaneous fission of U-238.
In order to determine a fission-track age, an aliquot of a uranium-bearing mineral is mounted in a small (less than 1" diameter) epoxy wafer, which is subsequently ground to expose the mineral grains and then polished. The polished surfaces of the mineral grains are then etched to reveal (i.e. enlarge) the naturally produced U-238 fission-tracks that have accumulated over geologic time. These etched tracks are needlelike features about 1-2 microns in diameter and about 15 microns in length. The polished and etched grain mounts are covered with thin sheets of uranium mica and placed in a tube that is irradiated with thermal neutrons in a nuclear reactor. Irradiation induces fission of U-235 in the mineral grains in direct proportion to its concentration and the neutron fluence. This fissioning of U-235 produces a second set of tracks termed induced tracks, which pass into the mica sheets wherever they are in contact with the polished surfaces of the mineral grains. Following irradiation, the mica sheets are separated from the grain mounts and etched to reveal these induced tracks.
In order to calculate the fission-track age, it is necessary to measure track densities of the surfaces of the mineral grains as well a mica helps in contact with the mineral grains. This is accomplished by counting tracks contained within a grid reticle in the path of the microscope, typically at magnifications between 1000.times. and 2000.times.. The reticle is first superimposed on a mineral grain selected at random in the mount, the number of tracks contained within the grid is determined by visual counting. Next, the reticle is placed over the complementary area comprising a set of complementary points) on the mica sheet that was in contact with the mineral grain during irradiation, and tracks within the grid are again counted. The process is repeated for several grains, until sufficient tracks are counted for a reliable age to be determined.
The placement of grain mounts and mica sheets in face-to-face orientations during irradiation produces counting areas that are mirror images, that is, counting areas that consist of complementary points. Thus, the problem of locating counting areas at high power reduces to the more general problem of parcentric positioning of a point on the grain mount with its complement on the mica sheet. In fission-track dating, parcentric position is a significant problem. In particular, complementary points on the mica sheet are extremely difficult to identify, because (1) there is no grain outline on the mica sheet, only a concentration of induced tracks where the sheet was in contact with the mineral grain during irradiation, and (2) the geometry of the counting area on the mica sheet is reversed (i.e. a mirror image) with respect to the counting area on the mineral grain because they were oriented face-to-face during irradiation. Precise positioning of the counting grid on the mica sheet is both tedious and time consuming, and positioning uncertainties may be a significant source of error in the age determination.
Two types of apparatus are presently used to achieve parcentric positioning for fission-track dating. One type of apparatus is a mechanical stage that attaches directly to the viewing platform of a microscope with one or more mounting screws, and which is capable of movement in two horizontal, orthogonal directions using hand-operated knobs. Parcentric positioning is done manually using mental pattern recognition. The positioning process is quite tedious and, in fact, precise positioning can be done only when the mica sheet contains high a real densities of tracks.
The other type of apparatus is a mechanical stage that attaches to a microscope in place of the standard viewing platform, and in which movement in two horizontal, orthogonal directions is controlled by two stepper motors and a joystick. Some versions of this type of apparatus incorporates a third stepper motor to control the vertical position of the mechanical stage for focusing. Parcentric positioning is done in software. However, this requires the initial alignment of each grain mount and mica sheet, which are usually cemented side-by-side on a glass microscope slide, on the mechanical stage by manually locating and entering the coordinates of several sets of complementary points into an algorithm. Although the algorithm speeds up the counting process considerably, the quality of the alignment depends entirely on the ability of the analyst too accurately locate complementary points on the grain mounts and mica sheets. Like the manual type of apparatus described above, precise alignment is possible only when the mica sheet contains high areal densities of tracks.